T h i s paper n o t t o be c i t e d without p r i o r r e f e r e n c e t o t h e a u t h o r s
INTERNATIONAL COUNCIL FOR THE
EXPLORATION OF THE SEA
C.M. 1975/ F : 28
D e m e r s a l F i s h (N) Committee
R e f . S t a t i s t i c s Committee
ON DISCARDING I N THE SOLE FISHERY AND PRELIMINAR OBSERVATIONS
ON SURVIVAL RATES OF DSSCAR9ED PLAICE AND SOLE I N 1 9 7 5
J.F. d e Veen, P.H.M. Huwae a n d M.S.S, L a v a l e y e
Netherlands I n s t i t u t e f o r Fishery Investigations,
H a r i n g k a d e 1 , Ymuiden, The N e t h e r l a n d s .
1, Introduction
A t t h e l a s t C o u n c i l M e e t i n g a r e s o l u t i o n o f t h e S t a t i s t i c s Committee
u r g e d member c o u n t r i e s t o i n i t i a t e o r c o n t i n u e r e s e a r c h i n t o t h e
c o l l e c t i o n o f d a t a on d i s c a r d s , e s p e c i a l l y o f s p e c i e s u n d e r q u o t a
r e g u l a t i o n , and o f s u r v i v a l o f d i s c a r d e d f i s h .
I n t h e N e t h e r l a n d s r e s e a r c h on d i s c a r d i n g i n t h e beam t r a w l f i s h e r y f o r s o l e h a s b e e n c a r r i e d o u t i n I 9 6 8 a n d 1 9 7 0 ( d e Veen a n d
Rodenburg, 1971). I n t h e f i r s t h a l f o f 1975 t h i s s t u d y h a s been
r e p e a t e d , I n a d d i t i o n t o t h e c o l l e c t i o n o f d a t a o n b o a r d o f comm e r c i a l s h i p s n e c e s s a r y t o e s t i m a t e t h e t o t a l a n n u a l number o f
d i s c a r d s o f t h e whole f l e e t , r e s e a r c h h a s b e e n s t a r t e d i n t o t h e
s u r v i v a l of discarded p l a i c e and s o l e . I n t h i s paper t h e r e s u l t s
o f t h e s e s t u d i e s w i l l b e d i s c u s s e d a n d compared w i t h t h e e a r l i e r
d i s c a r d s t u d i e s o f 1968 a n d 1970.
2. Methods
Data o n d i s c a r d s w e r e c o l l e c t e d on b o a r d o f a number o f f l a t f i s h
b e a m t r a w l e r s w i t h e n g i n e power r a n g i n g from 240 - 1 3 5 0 b.h.p,
and f i s h i n g on d i f f e r e n t grounds d u r i n g 12 t r i p s ( s e e f i g u r e 1 ) .
During each c r u i s e t h e l e n g t h d i s t r i b u t i o n of discarded and
l a n d e d f i s h h a s b e e n e s t i m a t e d b y s a m p l i n g a l i m i t e d number o f
h a u l s . By r a i s i n g t h e s e d a t a t o t h e t o t a l amount o f d i s c a r d s a n d
k e p t f i s h d u r i n g t h e whole c r u i s e ( e s t i m a t e d b y t h e s k i p p e r i n
b a s k e t s o f known w e i g h t ) t h e t o t a l l e n g t h d i s t r i b u t i o n o f a l l d i s c a r d s f o r t h e whole c r u i s e c o u l d b e a s s e s s e d .
S p e c i e s s t u d i e d a r e p l a i c e , s o l e , d a b , lemon s o l e , t u r b o t , b r i l l ,
c o d a n d w h i t i n g , A l t h o u g h t h e number o f s h i p s s a m p l e d i s r a t h e r
l i m i t e d we t h i n k t h a t t h e r e s u l t s r e p r e s e n t more o r l e s s t h e
whole D u t c h s o l e f l e e t i n t h e f i r s t h a l f o f 1975.
F o r s u r v i v a l s t u d i e s we u s e d t a n k s o n b o a r d o f t h e R.V. " T r i d e n s "
d u r i n g c r u i s e s , I n t h e s e t a n k s a l a r g e number o f f l a t b a s i n s ,
e a c h w i t h own c o n t i n u o u s s e a w a t e r s u p p l y , c a n b e i n s e r t e d ,
making i t p o s s i b l e t o s t u d y some 1 0 0 0 f l a t f i s h f o r a number
o f d a y s ( u n d e r c o n d i t i o n s a p p r o a c h i n g n a t u r e as much as p o s s i b l e as temperature of s e a water i s concerned),
S u r v i v a l o f d i s c a r d e d f i s h d e p e n d s l a r g e l y on t h e amount o f
damage o r i n j u r i e s i n f l i c t e d t o t h e f i s h d u r i n g t h e i r s t a y
i n t h e n e t d u r i n g t h e f i s h i n g o p e r a t i o n and i n a d d i t i o n t o
e f f e c t s o f b e i n g h a n d l e d on d e c k b e f o r e b e i n g t h r o w n o v e r b o a r d .
F o r s u r v i v a l s t u d i e s we g r a d e d t h e f i s h a c c o r d i n g t o a p p a r e n t
damage t o t h e body o f t h e f i s h i n t o 6 c a t e g o r i e s from complet e l y h e a l t h y a n d undamaged t o d e a d f i s h * I n t h e s u r v i v a l s t u d i e s we p u t f i s h e s
b e l o n g i n g t o t h e v a r i o u s damage c a t e g o r i e s i n s e p a r a t e tanks and followed t h e i r s u r v i v a l during four
days,
I n order t o apply the r e s u l t s of these survival studies to
t h e d a t a from t h e c o m m e r c i a l s h i p s , we u s e d t h e same g r a d i n g
i n t o damage c a t e g o r i e s i n a s s e s s i n g t h e damage o f t h e f i s h
c a u g h t by t h e s o l e c u t t e r s ,
3, Results
3 m1
s p e c l e s and p e r u n i t o f e f f o r t
The t o t a l number o f d i s c a r d e d f i s h by s p e c i e s p e r 100 h o u r s
f i s h i n g , b a s e d on a l l d a t a c o l l e c t e d , i s a s f o l l o w s .
Whiting
2.2
-
The l e n g t h d i s t r i b u t i o n s o f t h e d i s c a r d e d p l a i c e a n d s o l e p e r 1 0 0
h o u r s f i s h i n g a r e g i v e n below f o r t h e c o a s t a l a r e a ( 0
12 miles
from t h e c o a s t ) a n d t h e o p e n s e a 0 1 2 m i l e s )
For c a l c u l a t i n g t h e l e n g t h d i s t r i b u t i o n o f t h e d i s c a r d o f t h e t o t a l f l e e t
we h a d t o c o u n t t h e d a t a o f t h e o p e n s e a c a t e g o r y t w i c e .
-
For other species only the length
d i s t r i b u t i o n of d i s c a r d s of t h e t o t a l
N o r t h S e a p e r 100 h o u r s f i s h i n g i s g i v e n .
3.2 R e t e n t i o n o g i v e s o f s o l e and p l a i c e
I n f i g u r e 2 t h e r e t e n t i o n o g i v e s d e r i v e d from t h e p r e s e n t s t u d y f o r
s o l e a n d p l a i c e a r e g i v e n as s o l i d l i n e s , F o r c o m p a r i s o n t h o s e g i v e n f o r t h e two s p e c i e s i n 1970 ( d e Veen a n d Rodenburg, 1 9 7 1 ) , a r e
a l s o g i v e n , i n broken l i n e s .
I n t h e s o l e t h e r e i s h a r d l y a n y change s i n c e 1970. A s i n t h a t y e a r
t h e 5 0 % x e n t i o n l e n g t h i s a b o u t 22 cm, 2 cm below t h e l e g a l m i nimum s i z e o f 24 cm, t h u s r e s u l t i n g i n a f a i r amount o f u n d e r s i z e d
s o l e s n o t d i s c a r d e d . One s h o u l d e x p e c t more o r l e s s a k n i f e - e d g e
s e l e c t i o n i n s o l e , where on b o a r d e a c h s o l e i s measured p r e c i s e l y
before landing.
24 cm on b o a r d a n d
I n p r a c t i s e , f i s h e r m e n k e e p s o l e s from 20
s e l e c t t h e f i s h t o be banded l a t e r ,
I n t h e p l a i c e we s e e a d i s t i n c t change i n t h e r e t e n t i o n o g i ~ r e ,
i t i s now much s t e e p e r ( e v e n s t e e p e r t h a n t h e p r e s e n t s o l e - o g i v e )
The s e l e c t i o n o f p l a i c e t o b e k e p t c a r r i e d o u t b y hand i s much
b e t t e r now, b u t s t i l l a f a i r l y l a r g e number o f p l a i c e o f above
25 cm ( i n t e r n a t i o n a l l e g a l s i z e ) i s d i s c a r d e d . The 50% r e t e n t i o n l e n g t h i s now more o r l e s s i d e n t i c a l t o t h e Dutch minimum
l a n d i n g s i z e o f 27 cm.
-
-
3.3 The number o f f i s h by s p e c i e s d i s c a r d - e d a n n u a l l y by t h e
t o t a l Dutch s o l e f l e e t
The t o t a l number o f f i s h d i s c a r d e d i n t h e f i r s t h a l f o f 1975
h a s b e e n e s t i m a t e d b y r a i s i n g t h e numbers p e r 1 0 0 h o u r s f i s h i n g t o t h e t o t a l number o f f i s h i n g h o u r s o f t h e f l e e t ,
To a r r i v e a t a n a n n u a l f i g u r e we h a v e d o u b l e d t h e r e s u l t i n g
figures,
plaice
1 1 0 , 8 0 0 , 000
1 0 , 4 0 0 , 000
Whiting
274, 200, 000
2 3 , 3 0 0 , 000
19, 3 0 0 , 000
F o r p l a i c e a n d s o l e we c a n compare t h e p r e s e n t f i g u r e s w i t h
t h o s e g i v e n f o r 1967 and 1970.
Plaice
1967
1970
1975
48, 600, 000
1 1 6 , 5 0 0 , 000
1 1 0 , 8 0 0 , 000
Sole
9 2 2 , 7 0 0 % 000 ' 1
' 1 5 7 , 3 0 0 , 000 ' )
1 0 , 400, 000
)
r e c a l c u l a t e d from
the i n i t i a l values
g i v e n by
de Veen a n d Rodenburg,
The number o f d i s c a r d s i s a f u n c t i o n o f t h e s t r e n g t h o f t h e r e l e v a n t y e a r c l a s s . T h i s s t r e n g t h c a n b e e s t i m a t e d by means o f v i r t u a l p o p u l a t i o n a n a l y s i s . T h i s t y p e o f a n a l y s i s , however, f a i l s
t o g i v e r e l i a b l e d a t a f o r t h e most r e c e n t r e c r u i t i n g y e a r c l a s s e s .
3.4 D e g r e e o f damage o f d i s c a r d e d f i s h
I n o r d e r t o be a b l e t 4 u s e t h e s u r v i v a l r a t e of discarded f i s h
d e r i v e d from t a n k e x p e r i m e n t s ( t o b e d e s c r i b e d l a t e r ) i n e s t i m a t i n g o v e r a l l s u r v i v a l o f d i s c a r d s i n t h e c o m m e r c i a l f l e e t we
h a v e a s s e s s e d t h e d e g r e e o f damage t o p l a i c e a n d s o l e i n t h e
c a t c h e s o f t h e commercial c u t t e r s ,
The f o l l o w i n g q u a l i t a t i v e s c a l e h a s b e e n u s e d :
D e s c r i p t i o n o f damage
fish lively,
fish
fish
fish
fish
s l i m e l a y e r i n t a c t , no v i s i b l e
damage t o s k i n .
Lively,
s l i m e l a y e r n o t more i n t a c t ,
s l i g h t s c r a t c h e s on s k i n .
n o t s o l i v e l y , s l i m e l a y e r p a r t l y removed, l o s s
of scales near tail.
sluggish,
s l i m e l a y e r m o s t l y removed, many
s c a l e s missing.
sluggish,
s l i m e l a y e r l o s t , most s c a l e s
The r e l a t i v e f r e q u e n c y p e r damage s t a g e f o r t h e t o t a l D u t c h
fleet is:
F o r p l a i c e t h e a v e r a g e damage f o r i n c r e a s i n g l e n g t h o f d i s c a r d s
was:
/-
speeds
(6
k n o t s ) a s compared w i t h t h e s m a l l e r
T h e r e i s a s l i g h t d e c r e a s e i n t h e a v e r a g e amount o f damage w i t h
increasing length.
S o l e c u t t e r s w i t h a l a r g e e n g i n e power a r e a b l e t o d r a g h e a v i e r
beamtrawls
w i t h more c h a i n s o v e r t h e b o t t o m s a n d moreover f i s h
with higher ,
c u t t e r s ( 3 - 4,5 k n o t s ) . T h e r e f o r e we may exp e c t more damage t o f i s h i n t h e n e t s o f t h e l a r g e c u t t e r s t h a n
i n those of t h e l e s s powerful boats.
The f o l l o w i n g t a b l e s g i v e t h e a v e r a g e damage s t a g e p e r s h i p s category f o r p l a i c e and s o l e ,
Plaice
ships category
A
B
C
D
E
F
A
B
C:
D
E
F
Sole
ships category
The i n f l u e n c e o f t h e s t r o n g e r e n g i n e on t h e d e g r e e o f damage t o
the discards i s c l e a r l y v i s i b l e f o r plaice i n the s h i p ' s category
> 1000 b , h . p , a n d f o r s o l e i n t h e c a t e g o r i e s 5 0 0 - 1 0 0 0 b.h.p.
a n d > 1000 b , h s p .
However, t h e r e s u l t s a r e n o t c o m p l e t e l y c o m p a r a b l e b e t w e e n s h i p ' s
c a t e g o r i e s , b e c a u s e s h i p s w i t h d i f f e r e n t e n g i n e power may f i s h o n
d i f f e r e n t f i s h i n g grounds,
F o r t h e o t h e r d i s c a r d s p e c i e s s u c h a s c o d a n d w h i t i n g no d e g r e e
o f damage h a s b e e n e s t i m a t e d b e c a u s e t h e s e s p e c i e s h a v e h a r d l y
a n y c h a n c e o f s u r v i v i n g when d i s c a r d e d ,
3.5
3.5.1
S u r v i v a l s t u d i e s on p l a i c e and s o l e i n t a n k s on board "Tridens"
Mortality caused i n t h e n e t
On b o a r d o f " T r i d e n s " t h e m o r t a l i t y i n f l i c t e d on f i s h i n t h e n e t
d u r i n g f i s h i n g h a s b e e n s t u d i e d i n November 1 9 7 2 , A p r i l
May
1973 and J a n u a r y
F e b r u a r y 1975.
During t h e s e p r e l i m i n a r experiments t h e i n f l u e n c e o f t h e durat i o n of t h e h a u l of an o t t e r t r a w l w i t h beamtrawlnet and 5 c h a i n s
on t h e damage o f t h e f i s h a n d t h e n e t - m o r t a l i t y w a s e s t i m a t e d .
I n p l a i c e i t w a s a p p a r e n t t h a t t h e d e g r e e o f damage i n c r e a s e d
with increasing duration of the haul.
P l a i c e , g r a d e d by s t a g e o f damage a n d d u r a t i o n o f h a u l were t h e n
p u t i n s e p a r a t e t a n k s a n d examined t w i c e a day. M o r t a l i t y i n c r e a s e d w i t h i n c r e a s i n g d e g r e e o f damage a n d p e r d e g r e e o f damage m o r t a l i t y more o r l e s s s t o p p e d a f t e r t h r e e d a y s . The s i t u a t i o n on tLo t h i r d d a y w a s t h e n t a k e n f o r c o n s t r u c t i n g s u r v i v a l
c u r v e s ( f i g a r e 3) i n o r d e r t o r e l a t e t h e m o r t a l i t y o f t h e p l a i c e
-
-
t o t h e t i m e t r a w l e d , t h e r e b y u s i n g t h e i n f o r m a t i o n on damage c o l l e c t e d i n each of t h e h a u l s during t h e c r u i s e s o f "?ridens",
Thus i n November 1972 i n t h e N o r t h S e a w i t h w i n d f o r c e s 5 - 8
40% o f t h e p l a i c e from h a u l s o f 20 m i n u t e s would h a v e d i e d
l a t e r i f t h e y had been d i s c a r d e d immediately and t h e v a l u e s
f o r o n e a n d two h o u r h a u l s a r e 60% a n d 74% ( f i g u r e 4 ) .
I n t h i s p a r t i c u l a r c a s e b a d w e a t h e r c o n d i t i o n s may h a v e h a d
a n ill e f f e c t ,
I n A p r i l 1 9 7 3 a s u r ~ r i v a lc u r v e f o r s o l e c o u l d b e c o n s t r u c t e d
f o r t h e same s h i p , same n e t a n d t h e s a m e amount o f c h a i n s i n
% h e I r i s h S e a w i t h b e t t e r l i e a t h e r c o n d i t i o n s ( 2 - 5) a n d i s
g i v e n in f i g u r e 5,
The d a t a o f t h e ' l T r i d e n s T ' c a n n o t be a p p l i e d d i r e c t l y t o t h e
d a t a o f t h e c o m m e r c i a l s h i p s , f o r t h e r e a s o n t h a t "!TTrdens"
used a r e l a t i v e l y l i g h t - weight n e t w i t h only 5 chains.
However, t h e i l l - e f f e c t s o f i n c r e a s i n g d u r a t i o n o f h a u l s o n
t h e amount o f darnage t o t h e f i s h a n d a c o r r e s p o n d i n g dec r e a s i n g s u r v i v a l r a t e o f d i s c a r d s shown i n z h e " T r i d e n s "
e x p e r i m e n t s , u n d o u b t e d l y w i l l be o p e r a t i n g i n t h e commercial
f l e e t w i t h h e a v e r bearntrawls.
I n t h e f i r s t h a l f o f 1975 t h e r e l a t i v e f r e q u e n c y o f t h e durat i o n o f h a u l s b y D u t c h b e a m t r a w l e r s was h s f o l l o w s :
Dul-aleion o f
haul
1
11 , O
11/2
2
20,4
56,3
I
3
31/2
10,3
1.3
0.4
4
hours
0.2
%
T h i s shoivs t h a t i n p r a c t i s e we h a v e t o t a k e a c c o u n t o f a n a v e r age d u r a t i o n o f a haul o f 2 hours,
3,5,2
M o r t a l i t y c a u s e d b y e x p o s u r e on d e c k
I n F e b r u a r y 1975 s u r v i v a l s t u d i e s were i n i t i a t e d on t o t a l mortal i t y o f n e t and o f exposure on deck t o g e t h e r .
The d u r a t i o n o f t h e h a u l s o f " T r i d e n s " w i t h t h e same n e t a s bef o r e b u t w i t h 2 c h a i n s w a s 20 m i n u t e s , P l a i c e a n d s o l e g r a d e d t o
damage were l e f t i n b a t c h e s on d e c k f o r 20 a n d f o r 40 m i n u t e s ,
t h e r e a f t e r p l a c e d i n t h e s e p a r a t e t a n k s a n d examined t w i c e a d.ay
f o r t h r e e days,
S u r v i v a l i n t h i s e x p e r i m e n t a t t h e e n d o f t h e t h i r d d a y w a s as
follows:
40 m i n e e x p o s u r e
Sole
20 min
40 min e x p o s u r e
T h i s shows t h a t i n w i n t e r p l a i c e o f damage - c a t e g o r y A(undamaged)
do n o t show a n y e x t r a m o r t a l i t y f o r e x p o s u r e o n d e c k b u t t h a t a n
i n c r e a s e i n t i m e on d e c k l e a d s t o a n i n c r e a s e d m o r t a l i t y i n damag e d p l a i c e . I n t h e c a s e o f s o l e s u r v i v a l i s v e r y low i n t h e c a s e
o f 20 m i n u t e s e x p o s u r e a n d a l l s o l e s d i e d a f t e r a n e x p o s u r e o f
40 m i n u t e s on d e c k .
The d u r a t i o n o f t h e p e r i o d s i n w h i c h d i s c a r d s were l y i n g on d e c k
on b o a r d o f c o m m e r c i a l c u t t e r s was d u r i n g 1 9 3 h a u l s i n t h e 1 2
c r u i s e s concerned:
Thus on a v e r a g e d i s c a r d s were e x p o s e d o n d e c k f o r 15-20 m i n u t e s ,
En t h e a b o v e m e n t i o n e d e x p e r i m e n t s no p l a i c e a n d s o l e were p u t
i n t a n k s i m m e d i a t e l y , ~ a i t h o ? n t e x p o s u r e on d e c k , It i s t h e r e f o r e
n o t p o s s i b l e t o s p l i t t h e t o t a l m o r t a l i t y i n t o n e t - a n d expos u r e m o r t a l i t y , Net m o r t a l i t y p e r damage s t a g e i n c r e a s e d w i t h
i n c r e a s i n g d u r a t i o n o f t h e h a u l a s i s shown i n f i g u r e 3,
Thus n e t - m o r t a l i t y i n 2 h o u r s h a u l s a s u s u a l i n c o m m e r c i a l s h i p
w i l l b e h i g h e r t h a n i n t h e 20 m i n u t e s h a u l o f t h e a b o v e e x p e r i ment, I t i s o u r p l a n t o r e p e a t t h e s e e x p o s u r e e x p e r i m e n t s if
p o s s i b l e on b o a r d o f c o m m e r c i a l s h i p s .
Reference
de Veen, J,F,, a n d
Rodenburg, W,F, , 1971
-
D i s c a r d i n g i n t h e Dutch s o l e
f i s h e r i e s i n I 9 6 9 and. 1970
ICES CM 1 9 7 1 / ~ : 1 1 , mimeo,
2 PP*
posikio~sV I S / ~ & dur/hY
71cruises OFcommercc/a/
vesse/s /jl the 1stAaF o f 1975
Sole
Plaice
Surv/'va/ OFNorkh Sea P/a&
per degree d damye a5 a
funckion OFkhe duration
o f khe h a /
D
E
dqme o f damage
A
C
%
Fg.4
Suruiva/ OF dsca-d~d
Nor& Sea Plaice
as a funckion o f khe
durakion OFkbe had
Pidens nov. 72
Sur viva/
afker
3 days
90
60
30
dumkion of- haul
t
%
PO
Ay. 5
Survivd per degree OF
damage in Irish Sea 5
Sole
F/dens a~r-17
73
A
B
C
D
E
degree of
damzge